The present invention relates to a transmission device for dividing the torque between two coaxial gears, in particular for a planetary gear train for aeronautical applications.
As is known, in the field of aeronautical engines planetary gear trains are widely used to transmit motion and convert power between a turbo power unit (with high speed and low torque) and at least one propulsive element (with high torque and low speed), as they implement such function very efficiently while being lightweight and compact.
One known solution is described in European patent EP1113193 and comprises an input shaft and an output shaft, which are coaxial to one another along an axis, rotate about said axis and are coupled to one another by means of a planetary gear train. Said gear train has a planetary carrier or gear carrier structure, which supports two sets of planet gears. Each of the two sets of planet gears meshes with a respective sun gear, made to rotate by the input shaft.
The two sets of planet gears define two stages in which the torque from the input shaft is divided and follows separate torque transmission paths. In this way, the loads transmitted between the teeth of the gear train are lower than when the torque is transmitted to a single set of planets. Thus, the solution achieved is relatively compact, with no reduction in the overall power generated by the gear train.
The torque from the input shaft is transmitted and divided between the two sun gears by means of a transmission device comprising two hollow transmission shafts, which are coaxial to the input shaft and comprise respective intermediate portions fitted one inside the other and movable in relation to one another. The sun gears are formed integrally on a respective axial end of said transmission shafts.
At the opposite axial end, instead, the two transmission shafts end with respective flanges, which axially support one another and are fixed to a flange of the input shaft, so as to define a transmission coupling that causes both shafts to rotate. Therefore, when the torque is transmitted from the flange of the input shaft to the other two flanges, it is automatically divided between the two transmission shafts, according to the relative torsional rigidity between the two torque paths and the manufacturing and assembly tolerances of the device. Indeed, three conditions must be met in order for the loads to be distributed evenly across the gears and across the bearings of the gear train:
(i) the rigidity of the two torque paths must be perfectly balanced;
(ii) maximum precision must be guaranteed in the angular alignment (angular phasing) of the two sun gears; and
(iii) the teeth of the two sun gears must be equally spaced angularly about all 360°.
In use, the forces acting between the turbo power unit and the propulsive element could take the turbo power unit and the propulsive element out of alignment such as to affect the functionality and duration of the planetary gear train.
For that purpose, a known solution envisages the use of transmission shafts the length of which is much greater than the diameter thereof so that they are able to withstand any misalignment between the turbo power unit and the propulsive element, and prevent the transmission of such misalignment to the planetary gear train. However, this solution clearly has very large axial dimensions.
Therefore the need is felt to improve the solutions known in the prior art, of the type described above, in order to reduce their axial dimensions while preserving the capacity of the transmission shafts to absorb any misalignment between the turbo power unit and the propulsive element.
The purpose of the present invention is to produce a transmission device for dividing the torque between two coaxial gears, in particular for a planetary gear train for aeronautical applications, which satisfies the above need in a simple and economical manner.
According to the invention, this purpose is achieved with a transmission device for dividing the torque between two coaxial gears, in particular for a planetary gear train for aeronautical applications, as disclosed in claim 1.